1. Field of the Invention
The present invention relates generally to the field of wet milling of corn into useful products. More particularly, it concerns enhanced recovery of starch from corn wet milling.
2. Description of Related Art
Corn kernels contain starch, protein, water, fiber, and other substances, which can be separated to make various useful products. Starch is one component of the kernel, and the starch recovered from wet milling of corn can be sold as such or can be modified using chemical or biological means to produce a variety of products with higher commercial value.
The corn wet milling process is used to refine corn kernels into end products such as starch, oil, and feed. In general, kernels of corn are steeped in an aqueous solution under controlled conditions to soften the kernels and facilitate separation of the kernels"" components. After steeping, the aqueous solution, referred to as steepwater, is drawn off. The corn kernel, swollen as a result of the steeping, is then coarse-milled to allow removal of the germ. Oil is removed from the germ and refined to make corn oil. The remainder of the germ is dried to form corn germ meal, or it may be used as an ingredient in corn gluten feed.
After the germ is removed, the remainder of the kernel is milled again to pulverize endosperm particles while leaving fibrous material nearly intact. Fiber is separated from the starch and gluten by washing and screening. The fiber is then combined with the dried remains of the steepwater to make corn gluten feed.
The stream of starch and protein (e.g., gluten) that is separated from the fiber is first concentrated (e.g. thickened by removal of aqueous medium), and then the starch and protein are separated, in what is often referred to as the primary starch separation step, and the gluten is dried to form corn gluten meal. The thickening of the starch-protein stream and the primary starch separation steps are both typically done by centrifugation. The primary starch separation step yields two streams, (1) a stream that comprises the majority of the gluten from the thickened starch-protein stream (e.g. gluten stream) and (2) a stream that comprises the majority of the starch from the thickened starch-protein stream (e.g. starch stream).
The gluten stream from the primary separation step typically comprises about 3 to 5% (by weight) dry solids, which can be thickened and concentrated mechanically or by membrane filtration. The concentrated and dewatered stream is then sent to a gluten drier, to remove most of the remaining water (e.g., to about 90-95% dry solids). The dry end product is corn gluten meal.
The starch stream from the primary starch separation step is washed extensively, and fresh aqueous media is combined with the starch stream during the washing process. The amount of fresh aqueous media used in this washing stage determines the purity of the starch. Typically the starch stream comprises about 2 to 5% ds (dry solids) protein and about 12 to 17 g/L of soluble impurities when it enters into the starch washing operations. The protein content in the washed starch stream is typically reduced to less than about 1% ds by counter-current washing in a series of hydrocyclones. The aqueous wash media from the starch washing operation can be shunted from counter-current washing in the hydrocyclones to a clarifier centrifuge that is used to recover a relatively small amount of starch (e.g. typically between about 5 and 15% dry solids are recovered by clarification and these dry solids typically comprise starch, gluten, and impurities among other components) that was lost from the main starch stream during its washing. The starch recovered from the wash water is then combined with the starch-protein stream after the thickening step and before the primary starch separation step. Following washing the main starch stream comprising the majority of the starch is mechanically dewatered (e.g. with centrifuges or vacuum filters) to concentrate the starch. Some or all of the recovered starch can optionally be chemically or enzymatically modified or treated before being sold to the food, paper, or textile industries. Alternatively, the dewatered starch stream can be dried to produce dry cornstarch, which can be sold as such, or which can alternatively undergo further treatment or modification (e.g. thinning, among others) to obtain a final product.
A post-wet milling treatment of recovered starch involves hydrolyzing it to produce glucose (dextrose) and other oligosaccharides, which can in turn be used as carbon sources in fermentations from which products like ethanol and organic acids, among others, can be recovered. Furthermore clarified dextrose produced from starch can itself be sold as a sweetener or can be converted (e.g. via enzymatic treatment) to high fructose syrups, which can also be used as sweeteners. Alternatively, starch recovered from the corn wet milling that has been chemically treated (e.g. hydroxyethylated dent corn starches and thinned unsubstituted waxy maize starches) can be used in paper coating formulations to give the required rheology, water holding, and binding properties to the coating formulation.
Microfiltration and nanofiltration techniques have been used in the past in certain starch-related applications in which the starch had been treated or modified after recovery from the milling process. For example, European patent application 0,452,238 discloses using a nanofiltration membrane to filter a starch slurry, passing the dextrose in the slurry through the membrane while retaining the di-and trisaccharides, thereby producing a glucose syrup, which is about 95 wt % dextrose and 5 wt % di- and trisaccharides. European patent application 0,176,621 discloses a process for obtaining glucose from thinned starch. The process includes a separation step that produces a glucose-enriched stream and a glucose-depleted stream. Membrane-based separation is disclosed as one suitable separation technique.
It is desirable to enhance the recovery of starches in the corn wet milling process itself, prior to further modifications and treatments of the recovered starch. Thus, efficient means of concentrating and recovering the starch from various aqueous process streams are important to the overall economics of the corn wet milling process.
The present invention is directed to a corn wet milling process that comprises the steps of (1) separating wet milled de-germed corn particles into a fiber component and a first stream comprising water, starch, and protein (e.g. fiber separation step); (2) performing membrane filtration of the first stream (e.g. starch-protein stream thickening) producing a first retentate and a first aqueous permeate; (3) and separating the first retentate (e.g. thickened starch-protein stream) into a second stream and a third stream (e.g. primary starch separation step), wherein the second stream comprises water and a majority of the starch present in the first retentate, and the third stream comprises water and a majority of the protein present in the first retentate. The corn wet milling process of the present invention can comprise additional steps between membrane filtration and separating the first retentate. A majority in the present invention refers to an amount that is greater than 50% of the starting material. Thus, for example, the second stream comprises more than half of the starch that was present in the first retentate (thickened starch-protein stream).
The corn wet milling process of the present invention can further comprise washing the second stream (e.g. starch stream from the primary starch separation), which is rich in starch, with added aqueous media; and separating the washed second stream into a fourth stream and a fifth stream, wherein the fourth stream (e.g. washed starch stream) comprises water and a majority of the starch present in the second stream, and the fifth stream comprises greater than about 85 wt % water and less than about 15% dry solids. Thus, the fifth stream, or aqueous wash media, comprises water and small amounts of starch lost from the fourth stream, gluten, and impurities. Certain embodiments further comprise dewatering the fourth stream (e.g. washed starch stream) to yield a sixth stream (e.g. recovered starch stream). The sixth stream can undergo further treatments or modification or it can be dried to produce a dry recovered cornstarch.
Certain embodiments of the present invention further comprise membrane filtration (e.g. clarification) of the fifth stream (e.g. the aqueous wash media) to produce a second retentate (e.g. comprising recovered starch lost from the fourth stream) and a second aqueous permeate having less than about 2% dry solids (e.g. clarified wash media). The second retentate can be combined with the first retentate (e.g. thickened starch-protein stream) prior to the primary starch separation step, and the second aqueous permeate can be combined with the first stream prior to thickening (e.g. membrane filtration of the of starch-protein stream).
In certain embodiments of the present invention, the washed starch (fourth stream) can be treated with alkali to remove off-odor and flavor, particularly for starch that is intended for use in food. Preferably, the fourth stream has a dry solids concentration of between about 18% and 45% dry solids prior to adjusting the pH with alkali. The alkali treatment involves adjusting the pH of the fourth stream to within the range of 0.5 to 2 pH units below the pasting pH of the starch by addition of alkali to produce an alkali treated stream. Alkaline liquid can be removed or washed from the alkali treated stream (e.g. pH adjusted fourth stream) within about 5 hours of adjusting the pH. The alkali treated stream can be washed with aqueous media such that it does not alter the pH of the alkali treated stream to a pH below about 10 to produce a product starch stream. Preferably, washing occurs following removal of alkaline liquid from the alkali treated stream, but it can occur without prior removal of alkaline liquid. The product starch stream can be dried to produce a dry starch product, and if spray dried or drum dried, the dry starch product can be gelatinized. Chemical modification of starch can be performed on (1) starch present in the fourth stream before adjusting the pH with alkali or (2) starch present in the product starch stream (e.g. after pH adjustment and washing). Alkali treatment of starch can further comprise neutralizing the product starch stream (e.g. after washing step). Typically the washed product starch stream will have a pH of between about 10 and 12 prior to neutralization by the addition of a neutralizing agent that is used to adjust the pH of the product starch stream to between about 6 and 10.
Embodiments of the present invention permit the replacement of other separation equipment and techniques, such as centrifuges, with membrane filtration systems for the recovery and/or concentration of starch in certain aqueous streams in a corn wet milling process. Up to about 3xc3x97 concentration and possibly more of the dry solids in the first stream comprising starch and protein (e.g., gluten) can be achieved using membrane filtration.
Processes of the present invention have the following benefits: enhanced recovery of starch per unit amount of corn processed, thereby allowing the production of a greater amount of starch-based higher value products, and lower capital costs for the equipment needed to perform the process.